There is a separation membrane composed of a specific polymer compound, to perform in selective permeation of a desired gas component and thereby separating it. As specific industrial application thereof, relating to a global warming issue, studies have been carried out for separating and recovering carbon dioxide from a large-scale carbon dioxide source, e.g., a thermal power station, a cement plant, and a blast furnace in a steel plant. Such membrane separation technique purportedly attracts attention, for the solution of an environmental issue, as a means for achieving the operation with relatively small energy. Meanwhile, natural gas or bio gas (gases generated by fermentation and anaerobic digestion of excreta of organisms, organic fertilizers, biodegradable substances, polluted water, garbages, energy crops, and the like) is mainly composed of a mixed gas of methane and carbon dioxide. For removing carbon dioxide from the above described mixed gas, a gas separation method by using a membrane has been studied (see Patent Literature 1, Patent Literature 2 and the like).
On the other hand, in the mixed gas to which the separating operation above is applied, a considerable amount of moisture is often contained. The separation membrane to be applied there hence needs to be protected against the moisture. From this viewpoint, it is proposed to modify the surface of such a gas separation membrane to which a hydrophobic property is imparted (see Patent Literatures 3).